THE WA VE THEORY OF LIGHT. 325 



the light in the neighbourhood of that black 

 line A (Fig. 48) has for its wave-length 7-6 ; B 

 has 6-87; D has 5-89; the "frequency" for A is 

 3*9 times 100 million million, the frequency of D 

 light is 5'i times 100 million million per second. 



Now what force is concerned in those vibrations 

 as compared with sound at the rate of 400 vibra- 

 tions per second ? Suppose for a moment the same 

 matter was to move to and fro through the same 

 range but 400 million million times per second. 

 The force required is as the square of the number 

 expressing the frequency. Double frequency 

 would require quadruple force for the vibration 

 of the same body. Suppose I vibrate my hand 

 again, as I did before. If I move it once per 

 second a moderate force is required ; for it to 

 vibrate ten times per second 100 times as much 

 force is required ; for 400 vibrations per second 

 160,000 times as much force. If I move my hand 

 once per second through a space of a quarter of an 

 inch a very small force is required ; it would require 

 very considerable force to move it ten times a 

 second, even through so small a range ; but think 



